Could someone educate me as to why it is standard practice to define constants with a variable name rather than to just use the constant in the code?

IE, why is:

#define MY_CONSTANT "My constant"
? MY_CONSTANT

better than

? "My constant"

I understand that it may make it easier to change the value of the constant if the need arises during design, but if the constant is only used in one place in the application is there still an advantage?

Thanks for any enlightenment.